The invention relates to a method and a device for laser-beam welding which is especially suitable for deep welding a wide variety of materials.
U.S. Pat. No. 4,914,268 discloses a welding method in which at least two different electron or laser beams are directed onto a workpiece to be welded, it being intended in particular for large-format parts to be connected to one another. According to the teaching described there, the two or three different beams are intended to perform different tasks. For instance, a first beam is provided for producing the actual welded joint and then a second beam is to be used to smooth the weld formed on the surface and a third beam is to perform a recrystallization to reverse at least partially the change in microstructure induced by the heat input, in order to avoid undesired states of stress in the area of the weld.
The relatively highly concentrated energy input that is required especially during deep welding gives rise to problems in that there is formed in the intensely heated-up area, extending relatively far into the workpiece, a steam capillary in which the plasma produced by the heating, and consequently also gaseous components, are under increased pressure and are prevented from escaping from the actual workpiece by a relatively small opening for the steam capillary being formed on account of the high energy input into the workpiece in relation to the energy input on the workpiece surface, possibly leading to problems in the interior of the workpiece and to uncontrolled splashing out of the steam capillary.
A further disadvantage is represented by the stresses induced by the great temperature gradients which remain in the material even after welding, and in this case especially in the area of the weld. Owing to the stress gradient and in dependence on material-specific properties, the formation of weld imperfections, especially the formation of hot and/or cold cracks, may occur during solidifying or cooling down. To avoid such thermal stresses, it is customary to carry out an appropriate temperature treatment, in which the entire workpiece is warmed up under definite, material-specific temperature conditions and is cooled down again. However, for a preheating of the workpiece to be welded, likewise known for avoiding thermal stresses, this requires an increased amount of time, and in particular an increased amount of energy, which has adverse effects on effectiveness and costs.
It is therefore the object of the invention to specify a possible way in which a wide variety of materials can be welded, especially deep-welded, by means of laser radiation and at the same time a reduced amount of work can be achieved simultaneously with good quality of the welded joint produced.
This object preferably is achieved by the characterizing features of the present invention. Advantageous embodiments and further developments of the invention will be apparent from the description provided herein.
The solution according to the invention is then employed in such a way that, by suitable beam-shaping, two areas are simultaneously irradiated with different intensities by means of at least one laser beam, a small area being irradiated with a great intensity and the laser beam being shaped in such a way that the maximum beam intensity acts in the workpiece and not on its surface, and a larger area being irradiated with a lesser intensity on the workpiece surface. The irradiation with the great intensity has the effect of forming in the workpiece a steam capillary, the opening of which is widened in a bell-shaped manner in the workpiece surface by the second irradiation with lower intensity. The larger area, which is irradiated with the lesser intensity, also leads advantageously to the temperature gradients being reduced and the cooling rate of the melted material being reduced, so that the thermal stresses and their gradients in the welding area can be greatly reduced and it is possible to dispense with thermal pre- or post-treatment.
The bell-shaped opening of the steam capillary in the direction of the workpiece surface has the effect that gaseous components attempting to escape from this area can leave unhindered and no build-ups or splashes occur. This also avoids material separations in the area of the weld, which may otherwise occur due to gas bubbles entrapped in the solidifying melted material. The bell-shaped opening of the steam capillary and the resultant shape of the bath of melted material surrounding the capillary may lead to an avoidance of process instabilities during deep welding at high welding speeds (humping effect).
There are in principle two possible ways of carrying out the method according to the invention. One is for a laser beam of a laser-beam source to be split into two different partial beams. The two partial beams can then be shaped and widened differently and then impinge on the workpiece in superposed form.
A second possible way is to use at least two laser-beam sources, the respective laser beams of which are differently focused and act with different intensities on or in the workpiece. Laser-beam sources which already have different output powers may be used for this purpose, the laser-beam source that directs the laser beam for forming the steam capillary onto the workpiece also having the higher output power.
Examples of lasers which may be used for this purpose are a CO2 laser or an NdYAG laser, the beam of which is focused by a beam-shaping unit in such a way that the maximum intensity is obtained in the workpiece. The second laser may be a high-power diode laser, which may have an output power of approximately 1 kW or above.
In the method according to the invention, it is possible to work advantageously in such a way that the position of the area that is impinged upon with the greater intensity can be varied in relation to the other area, that is irradiated with the lower intensity on a material-specific basis or during the actual welding. The steam capillary can thus be formed for example in an area arranged off-center, within the area that surrounds this area and is irradiated with the lower intensity, if the laser beam used for this purpose is correspondingly aligned.
It may also be favorable if the ratios between the surface areas of the two different areas can be set, for example by modified beam-shaping, for example taking into consideration the material to be welded. This may be especially advantageous if the different thermal conductivities of the respective materials of the workpieces to be welded are taken into consideration.
The intensity of the laser radiation may, however, also be changed by regulating or controlling the laser power. In this way, on the one hand, the changing may take place with the material to be welded taken into consideration and, on the other hand, there is the possibility of regulating the laser power during the welding process. Such regulation can favorably take place in conjunction with a temperature measurement, the temperature distribution favorably being measured at least in the melted area. The temperature measurement should take place contactlessly, it being possible to use a plurality of individual temperature sensors that are locationally separated from one another or a correspondingly designed array. However, the known method of thermovision may also be used for temperature measurement. The regulation of the intensity with which the workpiece is irradiated by the various laser beams or partial beams may advantageously be varied separately for each individual beam.
A further possible way of bringing influence to bear while the method according to the invention is being carried out is for the position and/or the alignment of the individual laser beams or the partial beams of a laser beam to be changed in relation to one another by moving the laser-beam sources or mirrors. For example, the laser beam or beams intended to irradiate the area of lower intensity is or are directed at an inclination onto the surface of the material and an area on the surface of the material deviating from a circular shape is irradiated, forming an elliptical surface area of which the longer longitudinal axis points in the direction of the weld. For certain arrangements, however, it may also be favorable to obtain a two-dimensional formation turned through 90xc2x0 C. thereto, allowing a wider edge weld area to be warmed up.
The temperatures that can be achieved in the workpiece may, however, also be influenced by changing the respective focusings of the individual laser beams.
In an advantageous embodiment of an example of a device according to the invention in which two different laser-beam sources are used, with it being possible for one of the laser-beam sources to comprise customary CO2 lasers or NdYAG lasers and for the second laser-beam source to comprise a high-power diode laser, the high-power diode laser may be integrated into the beam-shaping unit of the first laser-beam source, which leads to a reduction in the required overall size of the device.
A further possibility for a device to be used for the invention, with two laser-beam sources, is that the partial beams emerging from the individual emitters of a diode laser are coupled into optical fibers and can be directed via the optical fibers in a targeted and locationally distributed manner onto the surface of the workpiece. This leads to greater flexibility in the arrangement of the diode laser, since the diode lasers currently available have to be arranged at a very limited small distance from the workpiece and problems can occur as a result in the case of complicatedly structured workpieces.
For increasing the distance between the diode laser and the workpiece, the laser beam emerging from the diode laser may also be directed through a light guide of relatively large dimensions in comparison with conventional optical fibers, for example a cylindrical body provided with a reflective coating on the outside surface, via which the laser beam can then be directed over a greater distance and preferably via a further lens onto the workpiece surface.
Already existing laser-beam welding installations may also be retrofitted with the invention, in that in the simplest case a high-power diode laser is additionally adapted to such a laser-welding installation.